1. Field of the Invention
The present invention relates to X-ray diffraction instruments, and particularly to X-ray diffraction instruments for detecting X-ray diffraction patterns two-dimensionally and estimating residual stresses of measurement objects.
2. Description of Related Art
X-ray diffraction instruments are used as a non-destructive inspection tool for measuring various material properties (such as crystallographic structure, composition and residual stress). Goniometers, zero-dimensional scintillation counters (SC), one-dimensional position sensitive detectors (PSD), etc. are commonly and widely used to obtain X-ray diffraction data (such as intensity and angle of diffraction). However, these instruments offer only zero-/one-dimensional diffraction data by a single measurement. Thus, a complicated actuator and a long total measurement time are needed to obtain sufficient diffraction data required for a thoroughly satisfactory material analysis.
To overcome this disadvantage, X-ray diffraction instruments including a two-dimensional X-ray detector which provide a larger amount of diffraction information in a shorter period of measurement time are used. Examples of two-dimensional X-ray detectors include two-dimensional position sensitive proportional counters (PSPC) and imaging plates (IP). Imaging plates are a type of ionizing radiation image detector in which a photostimulable phosphor such as BaFX:Eu2+ (X=Br, I) is applied on a support plate made of a plastic or the like.
JP-A 2000-146871 discloses a micro X-ray diffraction instrument and a method of measurement, in which a micro area of a specimen is irradiated with an X-ray beam and the X-ray beams diffracted by the specimen are detected by a two-dimensional X-ray detector. The two-dimensional X-ray detector used in this micro X-ray diffraction instrument is a cylinder made of a photostimulable phosphor, and is placed in such a manner as to surround the specimen. The specimen is tilted (e.g., by 45°) so that both the X-ray beams diffracted in directions tangential to the specimen surface and the X-ray beams diffracted in directions normal to the specimen surface can be detected by the photostimulable phosphor X-ray detector. By using the JP-A 2000-146871 X-ray diffraction instrument, sufficient X-ray diffraction data can be captured by the photostimulable phosphor detector by rotating the specimen around only one axis (normal to the specimen surface), which is advantageous over most conventional X-ray diffraction instruments requiring rotations about two axes. Thus, this X-ray diffraction instrument has the advantage of simple structure, high diffraction intensity and short total measurement time.
JP-A 2005-351780 discloses an X-ray diffraction instrument including a two-dimensional X-ray detector that provides transmission diffraction measurement. This X-ray diffraction instrument includes: a specimen table for horizontally holding a specimen; an X-ray emitter for irradiating the specimen with an X-ray beam; an arm for actuating the X-ray emitter in such a way that the incident angle of the emitted X-ray beam relative to the specimen is set at a desired angle from 0° to 90°; and a partially-open cylinder made of a storage (photostimulable) phosphor that surrounds the specimen table for detecting the X-ray beams diffracted by the specimen. The phosphor cylinder is placed in such a manner that its axis is perpendicular to the emitted X-ray beam. The phosphor detector portion of the cylinder barrel extends circumferentially from 180° to 360° as measured from the horizontal (parallel to the table surface) on the side of the X-ray emitter, and more preferably from 100° to 360°, and the other portion of the cylinder barrel is open. The JP-A 2005-351780 X-ray diffraction instrument provides transmission diffraction measurement as well as reflection diffraction measurement.
JP-A Hei 6 (1994)-317484 discloses an X-ray exposure system for micro-area stress measurement including (from upstream to downstream along the X-ray path): a slit and first (upstream) screen; a sample stage mounted on a rotatable goniometer; an imaging plate on a support that is mounted on an arm rotatable about the emitted X-ray axis; and a second (downstream) screen just in front of the imaging plate. The emitted X-ray beam passes through the slit and first screen and is incident on a micro-area (e.g., 100 μm to 1 mm square) of a sample, and several discrete X-ray diffraction arcs (each being a part of a Debye ring) obtained by changing the X-ray angle incident on the sample several times are exposed on the same single stationary imaging plate. According to this JP-A Hei 6 (1994)-317484, several discrete X-ray arcs can be detected by a single measurement with a high angular accuracy, thus enabling micro-area stress measurement of polycrystalline materials in a short period of time.
JP-A 2005-241308 discloses an X-ray diffraction system in which a measurement object (a railway rail) is irradiated with X-ray and an image of the X-ray diffraction ring from the measurement object is captured. This X-ray system includes: an X-ray emitter for emitting the X-ray and an X-ray detector for storing the energy of the X-ray diffraction ring and producing the image of the diffraction ring. The X-ray emitter and the X-ray detector are mounted on a holder in such a manner the X-ray incident angle relative to the measurement object is fixed at a single angle. According to this JP-A 2005-241308, the X-ray diffraction system can perform X-ray diffraction measurement simply and conveniently. Also, the system is easy to use, cheap to manufacture and portable. In addition, the physical condition (such as residual stress) of a measurement object (a railway rail) can be evaluated by comparing the diffraction rings of the measurement object and of a standard specimen (an iron standard powder).
However, the X-ray diffraction instruments of JP-A 2000-146871 and JP-A 2005-351780 require an actuator for adjusting the position and/or orientation of the specimen and/or the X-ray emitter, and thus have disadvantages of complicated structure and large size. In addition, the two-dimensional X-ray detectors used in the above disclosures are cylindrical in form, and surround a specimen for detecting the X-ray beams diffracted by the specimen. Therefore, there is some limitation on the size and shape of specimens measurable by these X-ray diffraction instruments. In general, specimens measurable by conventional X-ray diffraction instruments are limited to relatively small objects (such as laboratory samples).
Also, the X-ray exposure system for micro-area stress measurement of JP-A Hei 6 (1994)-317484 requires a goniometer for rotating a sample stage on which a specimen is mounted, and thus has a disadvantage of complicated structure and some limitation on the size and shape of specimens to be measured. Furthermore, JP-A Hei 6 (1994)-317484 describes that, in order to measure the micro-area stress of the measurement specimen, a standard powder is placed on the specimen and that the X-ray is irradiated to both the specimen and the standard powder at the same time. However, JP-A Hei 6 (1994)-317484 is silent to a fixing method of the standard powder to the specimen.
Recently, there has been an increasing demand for on-site non-destructive inspection of the conditions (such as material abnormality and deterioration) of structural components of large apparatuses used in various plants. As described in JP-A 2000-146871, JP-A 2005-351780 and JP-A Hei 6 (1994)-317484, most conventional X-ray diffraction instruments are large in size, and there is some limitation on the size and shape of specimens. Thus, conventional X-ray instruments are very difficult to use as a tool for inspecting structural components of large apparatuses both non-destructively and on-site.
The X-ray diffraction system of JP-A 2005-241308 has advantages in that the X-ray emitter can stably impinge X-ray on a large measurement object at a predetermined fixed incident angle, and any actuator for rotating the measurement object is not required. However, JP-A 2005-241308 does not describe any method for setting the standard specimen required for the residual stress measurement. This is probably because the JP-A 2005-241308 technology is practically limited to the top surface of a railway rail. Also, this disclosure does not describe any measure to prevent leakage of X-ray emitted from the X-ray emitter or scattered by the measurement object.
When X-ray diffraction measurement of large apparatuses in plants or other systems is performed on-site, the measurement surfaces (the surfaces to be measured) may often be vertical or face downward. In such cases, it is difficult to stably attach a standard powder on a measurement surface, thus making accurate measurement and estimation difficult. Also, contamination by foreign materials is unacceptable in some plants. In such environments, attaching a standard powder on a measurement surface without scattering the powder is a particularly important requirement. In addition, it is desirable for operator safety to prevent X-ray leakage.